The present invention relates to a reflective liquid crystal display device of lamination type, and more specifically, to a reflective liquid crystal display device of lamination type in which two or more liquid crystal panels each having a liquid crystal layer of cholesteric liquid crystal having different selective reflection wavelengths are laminated.
Recently, the technical field of electronic paper that can be displayed and held without a power supply and electrically rewritten has been rapidly developing. Electronic paper is aimed at realizing an extremely low power consumption capable of a memory display even if the power supply is turned off, a reflective display gentle to on eyes, which does not tire the eyes, and a flexible, thin display device like paper for applications such as electronic books, electronic news papers, electronic posters, etc. As display systems, the electrophoresis system in which charged particles are moved in air or liquid, the twist ball system in which charged particles classified by two colors are rotated, the organic EL system, and the bistable cholesteric liquid crystal system of selective reflection type that utilizes the interference reflection of a liquid crystal layer are being developed.
Among these various systems, the cholesteric liquid crystal system has an advantage of “memory function”, “low power consumption”, “colorization”, etc. In particular, the cholesteric liquid crystal system is overwhelmingly advantageous in producing a color display. In systems other than the cholesteric liquid crystal system, color filters classified by three colors need to be arranged for each pixel, and therefore, the brightness is ⅓ at the maximum, which corresponds to the three laminates, and those systems are not practical. In contrast to this, in the cholesteric liquid crystal system, colors are reflected by the interference of liquid crystal, and therefore, a color display can be produced just by lamination and there is an advantage in that a brightness of nearly 50% or more can be obtained. A color display device that adopts the cholesteric liquid crystal system is described, for example in Patent document 1 (Japanese Unexamined Patent Publication (Kokai) No. 2002-116461).
FIG. 1A and FIG. 1B are diagrams explaining the principle of a display by cholesteric liquid crystal. As shown schematically, the panel has a configuration in which a liquid crystal layer 1 is sandwiched and held between transparent substrates 3 and 4. Substrate 3 is the substrate on the display surface side. On the surface outside substrate 4, a black light absorbing layer 5 is provided. The cholesteric liquid crystal has two stable states: one state in which the liquid crystal forms layers parallel to the substrate surface; and another state in which the liquid crystal forms layers perpendicular to the substrate surface. The two states can be switched electrically and are characterized by bistability by which the two states can be held without supply of power.
As shown in FIG. 1A, when high voltage is applied to an electrode (not shown) provided on substrates 3 and 4, the spiral axes of liquid crystal molecules 2 linked spirally are oriented in the direction perpendicular to substrates 3 and 4 and a state is brought about in which layers are parallel to the substrate surface. This state is called a planar state. The liquid crystal layer in the planar state selectively reflects (selective reflection) light having a wavelength in accordance with the spiral pitch of the liquid crystal molecules, exhibiting a specific color and a reflective display is produced, which is a light state (reflective state). Presently, reflected light is either right-circular polarized light or left-circular polarized light depending on the direction of rotation of the spiral pitch.
A wavelength at which the reflection is maximum is expressed by the following expression, where n is the average refractive index of the liquid crystal and p is the spiral pitch.λ=n·p 
On the other hand, a reflection band Δλ increases according to the refractive index anisotropy Δn.
In contrast to this, as shown in FIG. 1B, when low voltage is applied to the electrode provided on substrates 3 and 4, a state is brought about in which the spiral axes of liquid crystal molecules 2 linked spirally are oriented in the direction parallel to substrates 3 and 4. This state is called a focal conic state. In the focal conic state, interference reflection does not occur, and therefore, the light incident on the device is transmitted and absorbed by light absorbing layer 5 of substrate 4, which is a dark state (transmitting state). In the reflective state, the light that is not reflected is just transmitted through the liquid crystal layer, and it is therefore possible to synthesize a reflective color by arranging liquid crystal layers that reflect different colors in the lower layer.
Because of interference reflection, light reflected in the light state differs depending on the wavelength. Because of this, it is possible to obtain panels of reflected light which exhibit red (R), green (G), and blue (B) by setting the spiral pitch of the liquid crystal.
FIG. 2 is a diagram showing an outline of a color cholesteric liquid crystal display device that has an enabled color display by laminating three panels. As shown schematically, in the order from the display surface side, a blue (B) panel 10B, a green (G) panel 10G, and a red (R) panel 10R are laminated and thus a liquid crystal display device 9 is configured. A drive circuit 11 is connected to the electrode of each panel via flexible cables 12B, 12G, 12R. By applying voltage to the electrode of each panel from drive circuit 11, it is possible to put a cell corresponding to the electrode of each panel into a light state and dark state, and thus an image can be displayed. Each panel comprises a matrix electrode and can produce a dot matrix display.
FIG. 3 is a diagram showing a sectional view of the liquid crystal display device 9 in FIG. 2. The electrode is not shown schematically. Each of panels 10B, 10G, 10R has a configuration in which each of liquid crystal layers 1B, 1G, 1R is sandwiched and held between transparent substrates 3 and 4 and the liquid crystal layer is sealed by a seal 6. Panels 10B, 10B, 10R are arranged in the order from the display surface side and panels 10B, 10G are adhered by a first adhesion layer 7 and panels 10G, 10R are adhered by a second adhesion layer 8. On the surface outside the substrate on the opposite side of the display surface side of panel 10R, a black light absorbing layer 5 is provided. In the following explanation, panel 10B on the display surface side is referred to as a first (blue) panel and its liquid crystal layer 1B is as a first (blue) liquid crystal layer, panel 10G next to the first panel is referred to as a second (green) panel and its liquid crystal layer 1G as a second (green) liquid crystal layer, and panel 10R next to the second panel is referred to as a third (red) panel and its liquid crystal layer 1R as a third (red) liquid crystal layer.
If first panel 10B is put into the light (reflective) state and the second and third panels 10G, 10R are put into the dark (transmitting) state, a blue display is produced. Similarly, if second panel 10G is put into the light (reflective) state and first and third panels 10B, 10R are put into the dark (transmitting) state, a green display is produced, and if third panel 10R is put into the light (reflective) state and first and second panels 10B, 10R are brought into the dark (transmitting) state, a red display is produced. Further, if first and second panels 10B, 10G are put into the light (reflective) state and third panel 10R is put into the dark (transmitting) state, a cyan display is produced, if second and third panels 10G, 10R are put into the light (reflective) state and first panel 10B is put into the dark (transmitting) state, a yellow display is produced, and if the first and third panels 10B, 10R are put into the bright (reflective) state and the second panel 10G is put into the dark (transmitting) state, a magenta display is produced. If all of the first to third panels 10B, 10G, 10R are put into the bright (reflective) state, a white display is produced and if all of the first to third panels 10B, 10G, 10R are put into the dark (transmitting) state, a black display is produced.
In the reflective liquid crystal display system using the cholesteric liquid crystal, the planar state is functions as the “light state” and the focal conic state functions as the “dark state” as described above. The brightness of the display becomes brighter as the reflectance in the planar state (light state) becomes greater and the contrast becomes higher as the transparency in the focal conic state (dark state) becomes greater.
As described above, the reflected light in the planar state will be either right- or left-circular polarized light, and therefore, reflectance is 50% at the maximum. As one method for realizing a highly bright, a method is known, in which the reflectance in a specific direction (in the direction of observation) is increased by applying an orientation regulating force to the liquid crystal interface, which regulates the orientation of liquid crystal molecules to align the spiral axes of the spiral pitch in the planar state, and causing the selective reflection light to have directivity.
As a method for applying an orientation regulating force, a method for forming an orientation film on the liquid crystal interface and performing a rubbing process on the surface of the orientation film, a method for forming a light orientation film on the liquid crystal interface and irradiating the surface of the light orientation film with ultraviolet light, or a hybrid method combining them is generally known. The orientation regulating force differs depending on whether or not an orientation film or a light orientation film is provided and it also differs depending on the density of the rubbing process on the orientation film or the irradiation strength of the ultraviolet light onto the light orientation film.
By applying the orientation regulating force onto the interface with a cholesteric liquid crystal layer, the selective reflection light in the planar state will become more directional. FIG. 4 is a graph showing a spectral reflectance in the color cholesteric liquid crystal display device in FIG. 3, which is measured in the perpendicular direction when the second green panel 10G is put into the light (reflective) state, the first blue panel 10B and the second red panel are put into the dark (transmitting) state, and illuminated with a incidence angle of 30 degrees. As shown schematically, by performing rubbing, the reflectance curve becomes sharper and the reflectance of the peak wavelength is 50% or higher.
However, if a high-density rubbing process is performed to increase the orientation regulating force, the spiral axis of the cholesteric liquid crystal in the planar state is substantially in the direction of substrate normal, and therefore, light is reflected in a mirror reflection manner, and a problem arises in that the visual angle becomes very narrow. In addition, if the orientation regulating force is made too strong, it becomes difficult to maintain the focal conic state and bistability will be lost.
In order to solve such a problem, in Patent document 1, a mixed state is described, in which a poly-domain state in which the spiral axes of the liquid crystal are tilted somewhat from the substrate normal and their orientations differ randomly and a mono-domain state in which the spiral axes of the liquid crystal are substantially uniform in the direction of the substrate normal coexist in the planar state. Specifically, the rubbing density is increased in the order from the blue panel, the green panel, and the red panel and further the rubbing density on the non-display surface side is made higher than the rubbing density on the display surface side in each panel. In this manner, the liquid crystal layer with a longer selective reflection wavelength is made to have a stronger orientation regulating force.